Both neonatal sound deprivation and neonatal conductive hearing losses result in smaller than normal brainstem auditory neurons in experimental mice. The proposed morphological experiments will use experimental mice to determine: 1) If neonatal sound amplification can maintain normal development of brainstem auditory neurons; 2) if forebrain auditory neurons in the thalamus and cerebral cortex are adversely affected by neonatal sound deprivation; 3) If cochlear function or structure are adversely affected by either neonatal auditory deprivation or conductive losses; 4) if the patterned organization of brainstem auditory neurons is affected by neonatal conductive losses; and 5) if the postnatal age of cochlear loss determines the type and degree of dystrophy in the cochlear nuclei. The results of these studies will demonstrate, at the anatomical level, the central effects of early hearing impairments. They will, therefore, be of considerable help in determining the appropriate medical and therapeutic care of infants with hearing impairments.